1. Field of the Invention
This invention relates generally to a method of joining telescoped metal tube members, and more specifically to coating at least one of the surfaces to be joined with a layer of solder before telescoping the members, and then simultaneously heating the members to reflow the solder while applying pressure radially and continuing to apply pressure progressively in an axial direction with a heated forming means such that an axial movement of solder from the joint area disperses an oxide layer and allows the solder to wet the uncoated surface and form a joint. In effect a fluxless joining is accomplished with the use of a heated crimping or forming means which is capable of progressively reducing the radial clearance between the surfaces to be joined to a minimum so that solder is forced hydraulically out of the radial clearance to effectively scrub oxide from the surfaces to be joined.
2. Description of the Prior Art
In joining aluminum surfaces or surfaces that are provided with a layer of solder, it is customary to remove the oxide coating from the surfaces to be joined so as to allow the joining alloy to wet the surface to be joined to provide a metallurgical joint. A common way of removing oxide from the surfaces to be joined is to employ a flux before or during the joining operation, however, fluxless joining has been found to be more advantageous.
Fluxless techniques are presently available which eliminate the oxide problem wherein the pretinned members to be joined are moved relative to each other at soldering temperatures to break up the oxide film and permit wetting by the solder. U.S. Pat. No. 3,633,266 -Taylor discloses one such method of joining aluminum tube members by fluxless soldering. In the method disclosed in the Taylor patent at least one of the members to be joined is coated with solder. The members are dimensioned to provide an interference fit when telescoped so that when the members are heated to reflow the solder, engagement of the surfaces breaks up or disrupts the surfaces of the coating and the oxide film thereon to assure a bond between the tubes. While the above method may be effective in removing the oxide film and provide a satisfactory joint, the means employed to impart motion may of necessity be special high cost equipment. Further, to insure that when the members are moved relative to each other, the coating remains in contact, and rubs together to break up the surface oxide of the members, it is required that the members to be joined be fabricated within critical dimensions.
Another attempt at fluxless joining of aluminum surfaces is disclosed in U.S. Pat. No. 3,680,200-Terrill et al wherein tube ends are telescoped and a solder insert is positioned adjacent the area to be joined. The joint is heated and a multi-directional positive ultrasonic energy is applied to the joint site causing the molten joining alloy to cavitate thereby disrupting the oxide film and allowing the joining alloy to wet the surfaces to be joined. As pointed out in the Terrill patent, it is critical that the annular gap between male and female members ranges from 0.002 inches to 0.006 inches around substantially the entire periphery. Maintaining these critical gaps could add extra burdens in their manufacture.
Another technique employed in fluxless soldering is the ultrasonic dip soldering in which ultrasonic energy is used to disrupt the oxide film. U.S. Pat. No. 3,831,263-Dzierski discloses a dip or immersion soldering technique using an ultrasonically energized bath. The above technique requires that the total joint area be immersed in the solder bath which in some instances is difficult or impossible in a complex heat exchanger. It should be noted that relative to the present invention the ultrasonic dip soldering may be employed as the coating operation.
Canadian Pat. No. 671,383-Bouton discloses a method of tinning a body of aluminum wherein the aluminum is heated and the solid solder is drawn across the surface to melt the solder and wet the aluminum surface, while floating the oxide layer to the top of the molten body and stirring the solder so as to disperse the oxide layer.
Another attempt at joining aluminum surfaces is disclosed in U.S. Pat. No. 3,180,022-Briggs wherein at least one of the surfaces to be joined is coated with a bonding metal. The surfaces to be joined are brought into intimate contact and then heated at a temperature between the eutectic formation temperature and the melting point of the aluminum members, until the members are joined by diffusion of the formed eutectic away from the interface into the members.